1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to various new cell designs comprising a cathode having a first cathode active material of a relatively low energy density but of a relatively high rate capability contacted to one side of a current collector and with a second cathode active material having a relatively high energy density but of a relatively low rate capability in contact with the opposite side thereof. The anode comprises lithium foil as a relatively thin metal strip, such as of nickel or stainless steel, devoid of perforations and serving as both a current collector and lead. The present cell designs are useful in applications where a premium is placed on increased energy density, such as in power sources associated with implantable medical devices.
2. Prior Art
Electrochemical cells are predominantly used as the power source for implantable medical devices such as cardiac pacemakers, defibrillators, neurostimulators, drug pumps, hearing assist devices, and bone growth devices. Such implantable medical devices generally comprise a housing containing circuitry for controlling functioning of the medical device and at least one electrochemical cell for powering the control circuitry. Depending on the application, the requirements on the cell's electrochemical properties, such as power capability and energy density, are very different. However, lithium-based cell systems are preferred as the power source for implantable medical device applications. Currently used ones incorporate various cathode active materials including silver vanadium oxide (SVO), carbon monfluoride (CFx), iodine, manganese dioxide (MnO2), thionyl chloride, a mixture of SVO/CFx, and a sandwich of SVO/current collector/CFx.
Regardless the cathode active material, a novelty of the present invention lies in the incorporation of a screen-less current collector in the anode. The screen-less current collector is in the form of a distal foil tab and a proximal extending foil portion meeting each other at a junction imbedded in the anode body. This structure replaces the conventional anode current collector screen. As will be discussed in more detail in this application, it is preferred that the screen-less current collector have extending portions “angularly radiating” from the center of the anode.
Cantave et al. in U.S. Pat. No. 6,443,999 discloses a primary lithium electrochemical cell which has a nickel tab placed at a bottom corner of the anode sheet. The tab is a small rectangular piece of metal of a length that is slightly greater than its width. Importantly, the tab disclosed by Cantave lacks angularly radiating portions or elements. That means this prior art collector structure may not necessarily be as efficient as the present current collector in directing current from the peripheral edges of locations of the anode to the tab. For that reason, the prior art tab of Cantave et al. could result in unreacted lithium being left behind after the cell has reached end-of-life discharge. Unreacted lithium is wasted and detracts from the cell's volumetric efficiency.
Thus, there is a need to develop new electrochemical cell systems having good performance characteristics suitable for powering implantable medical devices with lower manufacturing costs. The present cell systems comprising a metal strip devoid of perforations, but having arm of proximal portions emanating from a distal tab/current collector portion in a radial manner toward edges of the anode are believed to achieve these objectives.